whitepaper - self consumption or grid independence with the victron energy · pdf...

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www.victronenergy.com

Rev 02

Self-consumption and

Grid independence

with the Victron Energy Storage Hub

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1. Introduction

2. Three system alternatives

2.1. VE Storage Hub-1



3. Essential feature of the three system alternatives: GridAssist

4. Short description of the main components of the VE Storage Hub

4.1. Battery: lead acid or Li-ion, part 1.

4.2. MultiPlus and Quattro inverter/chargers

4.3. BlueSolar MPPT solar charge controller

4.4. PV inverter

5. Electric power consumption at home

Base load (category 1 loads)

Other plug-in appliances (category 2 loads)

Non movable loads (category 3 loads)

6. Efficiency of the Hub

7. The Hub for the grid connected home

7.1. Powering the base load with Hub-1 and a Li-ion battery

7.2. Base load plus other plug-in appliances (category 2 and 3 loads) powered with Hub-1

7.3. Powering the base load with Hub-2 or -3

7.4. Base load plus other plug-in appliances powered with Hub-2 or -3

7.5. What about dark and rainy wintertime?

8. The off-grid Hub

8.1. Micro-CHP

8.2. Diesel engine powered generator

9. Definition: the 100% PV array and the 100% battery

10. Cost

10.1. Self-consumption: optimal storage capacity

10.2. Off-grid: optimal storage capacity

10.3. Battery: lead acid or Li-ion, part 2

10.4. The PV array

10.5. Examples: cost of the major components

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1. Introduction Electrical power generated by the sun and/or wind, and actual power consumption never match.

The result is feedback of power into the grid when excess power is generated, and power needed

from the grid when power generation is insufficient.

As more solar and wind power comes on line, it becomes increasingly difficult and expensive to

ensure stability of the grid.

Intermediate energy storage is therefore rapidly becoming an essential tool to keep power

fluctuations on the grid within manageable limits.

Moreover, as feed-in tariffs are decreasing, the business case for a home energy storage system that

increases self-consumption becomes more solid every day.

Intermediate energy storage increases self-consumption of harvested solar and/or wind power.

The natural next step is 100% self consumption and independence from the grid.

The Victron Energy Storage Hub offers the solution, and several additional

benefits With tens of thousands of grid independent and grid interactive systems installed worldwide, we

have the experience and the products to design the optimal system.

• Battery The core of the Hub consists of the battery, which is charged in case of excess solar/wind

power and discharged when consumption exceeds production.

Tubular plate OPzS and OPzV lead acid batteries have proven to perform very well in grid

interactive as well as off-grid systems.

Alternatively, a Li-ion battery will be the preferred solution when high charge/discharge

efficiency, small size and low weight are important.

For more information see section 4.1 and 9.3.

• Grid friendly The Hub can be used to reduce peak demand from the grid (by discharging the battery) as

well as peak supply back to the grid (by recharging the battery).

For more information see section 9.1.

• Ride through a power outage Energy stored in the battery can be used to provide power to essential equipment during a

power outage.

• Grid independence With sufficient battery capacity and if needed a micro-CHP or back-up genset, complete

independence from the grid can be achieved.

• Flexible We do not offer one Hub, but three alternative configurations, each of which can be tailored

to specific requirements.

• Field upgradable

Additional solar/wind power and battery storage can be connected at a later stage.

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2. Three system alternatives

2.1. VE Storage Hub-1 Hub-1 is the highest efficiency solution when most of the energy produced has to be stored in the

battery prior to use.

It is also the simplest, most robust and lowest cost solution.

The BlueSolar MPPT charge controller uses solar power to charge the battery.

The stored energy is used by a MultiPlus or Quattro inverter/charger to supply AC power to the load

and to feed excess power back into the grid.

In case of a utility power outage, the Hub will disconnect from the grid and continue to operate as a

standalone system.

If power will be fed back into the grid an anti-islanding device that complies to local regulations has

to be added to the system.

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2.2. VE Storage Hub-2 This is the most practical solution to add battery storage to an existing grid connected PV system.

DC electrical power generated by the solar panels is converted to AC by a PV inverter connected to

the AC output of an inverter/charger.

The AC input of the inverter/charger is connected to the grid.

If power will be fed back into the grid an anti-islanding device may have to be added to the system,

depending on local regulations.

Power from the PV inverter is supplied directly to the load.

In case of insufficient PV power, the inverter/charger will supply additional power from the battery,

or from the grid.

In case of excess PV power the inverter/charger will use the excess power to recharge the battery,

and/or to feed power back into the grid.

In case of a utility power outage, the Hub will disconnect from the grid and continue to operate as a

standalone system.

Planning and commissioning of this solution is more complicated than Hub-1 due to interaction

between the inverter/charger and the grid inverter.

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2.3. VE Storage Hub-3 DC electrical power generated by the solar panels is converted to AC by a PV inverter connected to

the AC input of an inverter/charger.

The power from the PV inverter is supplied to the load through the inverter/charger.

In case of insufficient PV power the inverter/charger will supply additional power from the battery, or

from the grid.

In case of excess PV power the inverter/charger will use the excess power to recharge the battery.

Once the battery is fully charged the PV inverter will supply excess power to the grid.

If the PV inverter is fitted with an anti-islanding device according to local regulations, an anti-

islanding device is not needed.

In contrast to the Hub-1 and Hub-2 solution the PV inverter will shut down in case of a utility power

outage. The Hub will continue to supply the load until the battery is discharged.

3. Essential feature of the three system alternatives: GridAssist

Thanks to GridAssist the inverter/charger can be underrated compared to the expected maximum

power required by the load. With GridAssist, the inverter/charger runs synchronized with the grid

and whenever AC power required exceeds the capability of the inverter/charger, additional power

will be taken from the grid, thus preventing system shut down due to overload.

GridAssist-1

One solution is to run the inverter/charger synchronized, but not connected, with the grid. The

connection to the grid (by closing the back feed protection relay in the inverter/charger) is made in

case of:

- System overload. Additional power from the grid is used until the load has been reduced to a

level that can be managed by the inverter/charger.

- Excess PV or wind power available to be fed back into the grid (if allowed by local

regulations).

GridAssist-2

The alternative is to connect the Hub permanently to the grid. The inverter/charger will manage its

output to match the load so that the average power taken from the grid is zero, except of course in

case of overload or excess power to be fed back into the grid. Warning: stable grid voltage needed!

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4. Short description of the main components of the VE Storage Hub

4.1. Battery: lead acid or Li-ion, part 1.

Next to small size and less weight Li-ion (Lithium-iron-phosphate: LiFePO₄ or LFP) is an attractive

alternative to lead-acid in grid connected or off-grid systems because of efficiency and service life.

Efficiency

The round trip energy efficiency (discharge from 100% to 0% and back to 100% charged) of the

average lead-acid battery is 70 to 80%.

The charge process of lead-acid batteries becomes particularly inefficient when 80% state of charge

has been reached. Between 80% and 100% charge efficiency is often less than 50%. And these figures

become worse in case of high current charging or discharging.

The efficiency of a lead-acid battery comes nowhere near to Li-ion. The efficiency of a LFP battery is

around 92%, under all operating conditions. http://www.almaden.ibm.com/institute/2009/resources/2009/presentations/ChetSandberg-

AlmadenInstitute2009-panel.pdf

http://people.duke.edu/~kjb17/tutorials/Energy_Storage_Technologies.pdf

Efficiency of energy storage systems, from

http://catedrasempresa.esi.us.es/endesared/documentos/jornada_almacenamiento/Pet_Hall.pdf

Service life

The battery in a PV and/or wind off-grid system may suffer from starvation during weeks or even

months (wintertime). This is lethal for a lead-acid battery. The battery will fail prematurely due to

sulfation.

In case off an off-grid system with lead acid batteries the constant worry should therefore be battery

state of charge: whatever happens, the battery must be fully recharged regularly, and never be left in

discharged state during days or weeks.

In a grid connected system the battery can easily be recharged to the full 100% regularly.

Note:

For a discussion of the sulfation problem in solar applications, see for example

http://mnre.gov.in/file-manager/UserFiles/report_batteries_solar_photovoltaic_applications.pdf

(especially the photographs on page 18)

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The life span of a LFP battery does not depend on its state of charge, as long as the voltage per

battery cell is kept within (broad) limits. A Li-ion BMS (Battery Management System) will do just that,

and no battery care whatsoever will be needed.

For more about batteries see section 9.

4.2. MultiPlus and Quattro inverter/chargers VE inverter/charger range from 800VA to 10kVA single phase, and up to six 10kVA modules can be

connected in parallel. All models can be configured for three phase operation.

All MultiPlus and Quattro inverter/chargers can be programmed to seamlessly integrate into

Hub-1, -2 or -3.

4.3. BlueSolar MPPT solar charge controller The charge controller converts DC voltage from the solar array into a voltage suitable to charge the

battery. A number of BlueSolar controllers can be connected in parallel, the only limitation being the

maximum charge current of the battery (which is very high in case of Li-ion).

The efficiency of a BlueSolar MPPT charge controller exceeds 98%.

4.4. PV inverter The PV inverter converts DC voltage from the solar array into AC voltage suitable to power the AC

loads. In a system without battery, all surplus power will fed back into the grid, and shortage of

power will be supplied by the grid.

A PV inverter cannot function without an external AC power source/sink (ACpss). The PV inverter will

therefore shut down when there is no ACpss available (such as a stable grid, suitable inverter or

inverter/charger).

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5. Electric power consumption at home A list of the most common home appliances and the amount of electricity they use will help to size

the Hub. Minimum

Appliance Power On-time Energy/day summertime

base load for a

two person

household

Base load (category 1) Tropical aquarium with water heater 100 W 24 h 2400 Wh

High efficiency refrigerator 20 W 24 h 480 Wh 480 Wh

High efficiency freezer 20 W 24 h 480 Wh 480 Wh

(with DC permanent magnet compressor motor)

Average refrigerator 50 W 24 h 1200 Wh

Average freezer 60 W 24 h 1440 Wh

Plug-in chargers and standby loads 30 W 24 h 720 Wh 720 Wh

Modem 10 W 24 h 240 Wh 240 Wh

Ventilation 30 W 24 h 720 Wh 720 Wh

Electric space heater 2000 W 12 h 24.000 Wh

Hot water heater (boiler) 3000 W 2 h 6000 Wh

Central heating (on) and water heater (on) 130 W 8 h 1040 Wh (wintertime, gas fired)

Central heating (off) and water heater (on) 130 W 2 h 260 Wh 260 Wh

Central heating standby 10 W 24 h 240 Wh 240 Wh

High efficiency lighting 200 W total 6 h (winter) 1200 Wh

3 h (summer) 600 Wh 600 Wh

One 100W traditional incandescent lamp 100 W 6 h (winter) 600 Wh

3 h (summer) 300 Wh

Electric floor heating in the bathroom 1000 W 3 h 3000 Wh

Radio 30 W 3 h 90 Wh 90 Wh

LCD TV 50 W 3 h 150 Wh 150 Wh

Large plasma screen TV 300 W 6 h 1800 Wh

Personal Computer 100 W 3 h 300 Wh 300 Wh

Laptop 30 W 3 h 90 Wh 90 Wh

Range hood 150 W – 300 W 1 h 150 Wh 150 Wh

________

Total summertime base load, energy conscious two person household 4370 Wh

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Other plug in appliances (category 2) Vacuum cleaner 1000 W 30 m 500 Wh 500 Wh

(start-up power 2000 W or more)

Hair dryer 800 W 6 m 80 Wh 80 Wh

Electric jug from 1000 W to 3000 W Bringing 3 liter water to the boil 360 Wh

(energy needed to bring 1 liter water to the boil: 120 Wh)

Coffee maker 800 W 10 m 120 Wh 120 Wh

Other kitchen appliances (mixer, blender, etc) 100 Wh 300 Wh

________

Total other plug in appliances, energy conscious two person household 1360 Wh

Appliances always connected to the same socket (category 3) Washing machine, cold fill 2000 W heater plus 600 W motor 1000 Wh per load

Washing machine, hot fill, average 600 W (peak power) 400 Wh per load

Washing machine, hot fill, best in class 165 W 100 Wh per load http://www.fisherpaykel.com/admin/pdfs/pdf_usecares/4912_NZ_QuickSmart_WashSmart_UG_hi.pdf

Clothes dryer with electric heater 3000 W 3000 Wh per load

Clothes dryer with gas heater 300 W 300 Wh per load

Clothes dryer with heat pump 1350 W 1350 Wh per load

http://www.atcoenergysense.com/NR/rdonlyres/635CE05C-6BD3-4421-A1D0-

C54CE4DDF20A/0/ManagingElectricityatHomeWebVersion.pdf

Dish washer, average 2000 W 1100 Wh per cycle

Dish washer with hot fill 1200 W 400 Wh per cycle

http://reg.energyrating.gov.au/comparator/product_types/

Microwave 2000 W 200 Wh

Electric stove, peak power 8000 W

Average power during cooking session 2000 W 30 m to 1 h 1000 Wh to 2000 Wh

Electric oven from 2000 W to 4000 W peak 30 m 2000 Wh

Swimming pool pump 700 W 8 h 5600 Wh

Water well pump 700 W 3 h 2100 Wh

Heat pump heating or cooling (airconditioning) can be 10 kWh per day or more

Table 1: Electric footprint of some common home appliances

Base load (category 1)

Some loads will nearly always be present: together these are the base load of the home.

All base loads may be on simultaneously.

It is not easy to reduce the base load. One could insert timers to completely shut down a number of

loads during the night and save at most 1 kWh (1 kWh = 1000 Wh).

Because of increased lighting and heating the wintertime base load is substantially higher than the

summertime base load.

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From table 1:

The reasonable minimum summertime daily base load is 4370 Wh

Peak power to be expected is 660 W

And average power 182 W

During wintertime (in a temperate climate) more lighting and central heating will increase the

minimum base load to 5750 Wh

Peak power does not increase 660 W

But average power does increase 240 W

A larger house and/or more people can easily increase the summertime base load to 8000 Wh

And in winter 11.000 Wh

Note:

In a small office or workshop the base load may even be relatively much higher (during working hours) in

comparison to the other loads.

Other plug-in appliances (category 2)

Plug-in appliances can be plugged into any socket anywhere in the home. This is especially true for

the vacuum cleaner. It is therefore virtually impossible to separate the base load from, especially, the

vacuum cleaner with its 1000 W running power and often much higher startup power.

But it is improbable that all plug-in appliances will be used simultaneously.

Appliances always connected to the same socket (category 3)

In most European homes the washing machine and dish washer are cold fill, and the clothes dryer is

with electric heater. If used every second day, and not simultaneously, they represent a 3 kW peak

load, and together with the microwave only 3 kWh energy required per day, average.

It is often possible to rearrange the wiring so that these loads can be completely separated from the

base load and other plug-in appliances.

And one can easily prevent them from being on simultaneously.

Note:

Table 1 shows that a lot can be done to reduce electric energy and (peak) power needed for these appliances.

The classification of loads in three categories does lead to interesting insights and helps discussing

the possibilities and limitations of self-consumption or off-grid operation.

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The electric footprint of the three load categories is summarized in table 2 below.

Table 2: Energy and power per load category

Notes:

1. In case of the two person energy conscious household the most efficient appliance alternatives have

been chosen.

2. The average home is inhabited by a family with two children, and fitted with the electric equipment as

found in the average European home today.

3. The above average home is about maximum comfort and luxury, including an electric induction

cooktop. Heat pump heating and/or cooling (airconditioning) have been left out: a case by case

approach is needed because of high energy consumption.

4. In all examples it has been assumed that high power equipment is not in use simultaneously.

As is clearly shown by the pie charts derived from table 2, the energy and therefore also the average

power needed for the base load (blue) is more than two thirds of the total.

Load Two person energy The average home Above average

category conscious household

Energy Peak Average Energy Peak Average Energy Peak Average

per day power power per day power power per day power power

Wh W W Wh W W Wh W W

Base load (summer) 4.370 660 182 8.380 1.305 349 18.960 2.560 790

Other plug-in appliances 1.360 2.000 57 1.640 2.000 68 1.920 2.000 80

Appliances always 350 1.200 15 2.050 3.000 85 7.100 13.100 296

connected to

the same socket

Total (summer) 6.080 3.860 253 12.070 6.305 503 27.980 17.660 1.166

Additional wintertime

base load 1380 0 58 2760 0 115 4140 0 173

Total (winter) 7.460 3.860 311 14.830 6.305 618 32.120 17.660 1.338

The two person efficient home

Energy/day

Base load

Other plug-in

appliances

Appliances always

connected to the

same socket

The average home

Energy/day

Above average

Energy/day

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But when looking at peak power required, the base load (blue) is always less than 30% of the total!

In other words: the peak to average load ratio of the base load is much lower than the other two

categories, see table 3.

Table 3: The peak to average load ratio of the three load categories

Conclusion The base load could be powered from the battery with a 1200 VA to 3 kVA inverter.

Category 2 and 3 loads need much more (peak) power when in use, and therefore a more powerful

inverter. But they are used during short periods of time only and the resulting energy/day required is

low. An inverter supplying the complete home (i. e. all load categories) will therefore operate most of

the time at only a few % of its rated power.

In case of the grid connected home it would therefore be quite rewarding to power only the base

load with an inverter, and connect the other loads to the grid.

In case of the off-grid home the grid is not available to assist when power hungry appliances are

switched on. More inverter power will therefore be needed.

Using electricity to generate heat (washing, drying, cooking) is expensive. Gas and/or solar water

heating are lower cost alternatives.

Load Two person energy The average home Above average

category conscious household

Peak/average load Peak/average load Peak/average load

Base load (summer) 3,6 3,7 3,2

Other plug-in appliances 35,3 29,3 25,0

Appliances always 82,3 29,3 42,6

connected to

the same socket

Total (summer) 15,2 11,5 14,7

The two person efficient homePeak power required

Base load

Other plug-in

appliances

Appliances always

connected to the

same socket

The average homePeak power required

Above average

Peak power required

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A load management system that would switch loads on when the sun is shining can improve self

consumption. The loads that that come to mind (see table 1) are: Hot water heater (boiler) Swimming pool pump

Water well pump

Washing machine

Clothes drier

Dish washer

But except for the pumps, the better solution is to first reduce electric energy required by these loads

by using hot fill (using thermal solar and/or gas heating).

6. Efficiency of the Hub

The Hub sits in between the solar/wind supply and the load. Unfortunately some energy will be lost

in the Hub. The losses are not negligible. The purpose of the calculation below is show were these

losses come from (answer: the battery!).

The hasty reader can skip the calculation and move directly to the conclusion.

The energy harvested Eh should cover the energy El consumed by the load, plus battery charge/discharge

losses, power conversion losses and losses in cabling and fuses.

6.1. If all harvested energy is stored in the battery prior to use

In case of Hub-1, if 0% of the harvested energy is consumed directly by the load (100% of the harvested energy

is stored in the battery prior to use), the resulting approximate efficiency Ƞ₀ = El / Eh is:

Ƞ₀ ≈ Ƞi x Ƞb x Ƞm x Ƞw

With for example:

94 % AC to DC conversion efficiency of the inverter/charger Ƞi ≈ 0,94

92% Li-ion battery efficiency Ƞb ≈ 0,92

98% MPPT charge controller efficiency Ƞm ≈ 0,98

2% losses in cabling and fuses Ƞw ≈ 0,98

The result is: Ƞ₀ ≈ 0,83

With a lead acid battery (Ƞb ≈ 0,8 or lower, see section 4.1)

The result is: Ƞ₀ ≈ 0,72 or lower.

And in case of Hub-2 or -3:

Ƞ₀ ≈ Ƞc∙Ƞi∙Ƞb∙Ƞpv∙Ƞv

With:

94 % AC to DC conversion efficiency of the inverter/charger: Ƞc ≈ 0,94

94% DC to AC conversion efficiency of the inverter/charger: Ƞi ≈ 0,94

92% Li-ion battery efficiency: Ƞb ≈ 0,92

97% PV inverter efficiency: Ƞpv ≈ 0,97

1% losses in cabling and fuses: Ƞv ≈ 0,99

The result is: Ƞ₀ ≈ 0,78

With a lead acid battery (Ƞb ≈ 0,8 or lower, see section 4.1)

The result is: Ƞ₀ ≈ 0,68 or lower.

6.2. If 40% of the harvested energy is consumed directly by the load The efficiency Ƞₓ will be higher if some of the harvested energy is consumed directly by the load.

In case of Hub-1:

Ƞₓ ≈ Ƞi∙(Xd+Ƞb∙(1-Xd))∙Ƞm∙Ƞw

Where Χd is the direct consumption factor.

Χd = 1 if all energy is consumed directly, without intermediate storage, and

Xd = 0 if all energy is stored prior to use.

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If 40% of the harvested energy is consumed directly by the load: Χd = 0,4 and Ƞ₄₀ ≈ 0.86 (with a Li-ion battery)

And in case of Hub-2 or -3:

Ƞₓ ≈ (Χd+Ƞc∙Ƞi∙Ƞb∙Ƞc∙(1-Xd))∙Ƞpv

With 40% of energy consumed directly by the load: Χd = 0,4 and Ƞ₄₀ ≈ 0.86 (with a Li-ion battery)

Notes:

1. Clearly, if a substantial percentage of the harvested energy is consumed directly by the load, the most

dramatic efficiency improvement is achieved in case of Hub-2 and-3 because direct consumption not

only bypasses the battery, but also the inverter/charger. In practice the improvement will be less

pronounced because Ƞc and Ƞi are load dependent and decrease when the average load of the

inverter/charger becomes low.

2. As indicated in note 1, the efficiencies of the devices constituting the Hub are not constant.

The inverter/charger will have a low efficiency at low loads, and maximum efficiency at about 75% of

its nominal output power. No load loss is about 1% of nominal output power.

The PV inverter and solar charge controller perform better at low loads, with no load losses of

approximately 0,2% and 0,05%.

Losses in cabling and fuses are proportional to the square of the current flowing through them,

resulting in rapidly increasing losses (= decreasing efficiency) at high loads.

In fact the efficiency of the Li-ion battery is the most constant of all, being virtually independent of

charge/discharge current and state of charge.

3. In case of power from the sun, in most homes direct consumption by the load will be much lower than

40%. Especially if all leave the home to go work or school in the morning and come back late in the

afternoon, nearly all consumption (except for the refrigerator and freezer) will take place when PV

input is zero.

Only when someone stays at home, or in case of a small office, hotel or other business, 40% direct

consumption or better can be achieved.

Hub-1 will therefore nearly always be the most efficient solution for the PV powered home.

6.3. Conclusion

Due to continuous variation of the load during the day and from one day to the next, it is not possible

to accurately calculate the efficiency of the Hub. Moreover, as PV or wind input are in general also

subject to wild variations, accurate efficiency calculations are a futile exercise.

In the following examples, 85% efficiency is assumed for systems with a Li-ion battery, and 75% for

systems with a lead acid battery.

7. The Hub for the grid connected home

7.1. Powering the base load with Hub-1 and a Li-ion battery

In case of a holiday cabin, small office or home without any category 2 and 3 loads, or if the base load

can be separated from all high power appliances (a big if, because an existing home will have to be

rewired, and careful planning of the wiring in a new home will be needed) a 800 VA to 3000 VA

inverter/charger will be the right choice.

7.1.1. Li-ion battery

If the requirement is to store enough energy to power the base load during one full summer day,

4,4 kWh to 19 kWh of stored energy will be needed (see table 2 or table 6-8 in section 9), plus 6%

conversion loss (in the inverter/charger) and plus 20% in order to limit Li-ion battery discharge to

80% (see section 9.3 for max discharge level of batteries).

The total energy storage capacity needed therefore ranges from 5,8 kWh (the two person energy

conscious household) to 25 kWh (the above average home).

The capacity of a 24 V Li-ion battery should therefore range from 240 Ah to a whopping 1000 Ah.

Better to move to 500 Ah at 48 V in the latter case (see table 8). The battery will not be more

expensive, but DC cabling will be cheaper and less cumbersome, and the charge controller will

produce two times more power at the same output current.

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Notes:

- Energy stored in the battery: E (kWh) = Ah x V x 1000.

- In practice not all energy produced during the day will be stored. A certain amount will be consumed

directly by the load, resulting in less than 80% battery discharge.

- About cable cross section: cable losses are proportional to R∙I². The current I becomes two times lower

when moving from 24 V to 48 V so that cable cross section can be reduced by a factor of four.

7.1.2. Solar array

Here a lot of parameters come into play: suitable surface available, local climate, can excess power

be fed back into the grid, etc.

Note:

The solar irradiation on sunny summer days on south facing panels with axis tilt ≈ latitude is roughly

8 kWh/m²/day, and relatively independent of latitude.

The average irradiation during a sunny summer month is 6-8 kWh/m²/day.

http://rredc.nrel.gov/solar/pubs/redbook/

A solar panel delivers its nominal output power (Wp) at 25℃ and 1000 W/m² irradiation.

In a laboratory the daily output of a 1 kWp PV array irradiated at 8 kWh/m²/day will therefore be 8 kWh.

In practice, due to less than perfect orientation, high panel temperature, and particulate build up on the

panels, the output of a 1 kWp PV array irradiated at 8 kWh/m²/day will be some 25% lower: 6 kWh instead of

8 kWh.

The assumption in the calculations in the following paragraphs is therefore that on a sunny summer day a

1 kWp array is irradiated with 8 kWh/m²/day and will produce 6 kWh/day, almost all over the world.

https://www.nvenergy.com/renewablesenvironment/renewablegenerations/documents/PVPerformanceSum

mary.pdf

http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

The average daily output of a solar array will of course depend on the local weather and be lower and often

much lower than the sunny summer day output: see table 4.

*The worst month in terms of PV output on the Northern Hemisphere

Table 4: Showing the dramatic reduction of PV output as a function of latitude

If, for example, the requirement is to harvest sufficient energy to supply the base load during a sunny

summer day, a 850 Wp array will be needed for the two person energy conscious household and

around 3700 Wp for the above average home (see table 6-8).

Average Average annual output/ Average December* day/

Latitude City annual output sunny summer day sunny summer day

kWh/kWp

60 Helsinki, Finland 800 39% 4%

61 Anchorage, AK 800 38% 6%

52 Amsterdam, Netherlands 900 43% 14%

48 Muenchen, Germany 1000 46% 18%

47 Seattle,WA 1000 46% 18%

43 Marseille, France 1500 68% 41%

41 New York, NY 1250 58% 35%

37 Sevilla, Spain 1600 74% 50%

34 Los Angeles, CA 1500 70% 63%

33 Phoenix, AZ 1750 81% 61%

26 Miami,FL 1400 65% 56%

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7.1.3. Battery charging

A Blue Solar MPPT 150/70 will take care of a 850 Wp array, together with a 24 V battery

(850 Wp*Ƞm*Ƞw / 24 V = 34 A charge current needed).

With a 3700 Wp array a 48 V battery is the better choice, and even then two MPPT 150/70

controllers will be needed (3700 Wp*Ƞm*Ƞw / 48 V = 74 A charge current needed).

7.1.4. Percentage of electric energy consumption covered by PVwhen powering the base load with

Hub-1 and a Li-ion battery

As can be concluded from table 2 this simple and relatively low cost solution will supply more than

70% of the electric energy needed per day, at least during sunny summer days.

And because PV output will never exceed consumption, feedback into the grid is not needed.

Note:

Depending on latitude and local climate a rough approximation of the average percentage of electric energy

consumption covered by PV over the year can be calculated as follows:

Total annual electric energy consumption (see table 6-8):

Ey = 365*(summertime consumption + wintertime consumption)/2

Average annual usable PV output (see table 4): Eypv = kWp*(average annual output)*(efficiency of the hub)

Percentage covered by PV: α (%) = 100*Eypv/Ey

Taking for example the average home, in Sevilla (Spain) or in Amsterdam (The Netherlands):

From table 7: Ey = 4788 kWh

From table 4: Eypv = 1,643*1600*0,85 = 2234 kWh (Sevilla) and 1,643*900*0,85 = 1257 kWh (Amsterdam)

Percentage covered by PV: α = 100*2234/4788 = 47% (Sevilla) and 26% (Amsterdam)

7.1.5. How much self-consumption?

If the solar array is dimensioned to never harvest more energy than required by the base load (plus

losses), 100% self-consumption will be achieved.

A lower capacity battery may result in some excess solar energy (once the battery is fully charged).

This excess could be fed back into the grid.

Alternatively the solar array could be downsized in order to match battery capacity.

7.1.6. What happens in case of a discharged battery (wintertime, bad weather)?

The inverter/charger will transfer the load to the grid (interruption free) and shut down. The

inverter/charger can be configured to restart after the battery has been partly or fully recharged by

the sun and/or wind.

A lead acid battery should not be used in partially discharged state for long periods of time.

A regular full recharge, using power from the grid or a generator is a necessity.

7.1.7. What happens in case of excess production?

This could happen when the home is vacant during the holiday period for example.

Excess power can be fed back into the grid.

If feedback into the grid is not possible, the charge controller will limit power taken from the solar

array, after the battery has been fully charged.

18

7.2. Base load plus other plug-in appliances (category 2 and 3 loads) powered with Hub-1 The simple set up described in the previous section can easily be upgraded to a more performing

system by using the GridAssist function.

The maximum AC power pass-through capacity of the MultiPlus models 800, 1200 and 1600 is 3,6 kW

(16 A at 230 V). At 2 kVA and above models with 6,9 kW or more pass-through capacity are available.

Category 2 loads can therefore be supplied, with some help from the grid. In case of sufficient pass-

through capacity the high power category 3 loads could also be supplied by the MultiPlus or Quattro,

with help from the grid.

Alternatively, category 3 loads could be wired directly to the grid, bypassing the MultiPlus or Quattro

(assuming a single phase grid connection), or could be connected to another phase (in case of a three

phase grid connection). Due to the very short on-time of category 3 loads, bypassing the Hub is a

practical solution with limited influence on self-consumption performance.

Unfortunately bypassing the Hub is not easily feasible with category 2 loads as they are frequently

moved from one AC socket to the other (especially the vacuum cleaner).

Note:

Multiplus or Quattro

The Multiplus has one AC input whereas the Quattro has two AC inputs with integrated transfer switch.

The Quattro can be connected to two independent AC sources, for example the grid and a generator,

or two generators. The Quattro will automatically connect to the active source.

7.2.1. Battery etc

The daily energy required by category 2 and 3 loads is low compared to the base load (see table 2).

Battery capacity and PV power therefore must be increased by some 25% to also power these loads

on a sunny summer day.

19

7.2.2. Percentage of electric energy consumption covered by PV

On a sunny summer day approximately 100% of the electric energy needed per day will covered.

And a rough approximation of the average percentage of electric energy consumption covered by PV

over the year can simply be read from table 4 and corrected for losses:

Li-ion battery: 0,85*74% = 63% for Sevilla and 0,85*43% = 37% for Amsterdam.

OPzS battery: 0,75*74% = 56% for Sevilla and 0,75*43% = 32% for Amsterdam.

7.2.3. How much-self consumption?

Only if carefully planned the power consumption of category 2+3 loads will be relatively constant

from day to day. Some excess energy may therefore be available on certain sunny summer days, and

a shortage on others.

7.3. Powering the base load with Hub-2 or -3

Instead of the solar charge controller, the inverter/charger now charges the battery.

The consequence is that the required charge current may be the determining factor to size the

inverter/charger.

A 850 Wp solar array is needed to power the base load of the two person energy conscious home on

a sunny summer day (see section 7.1). The resulting maximum charge current (when all harvested

power is used to charge the battery) at 24 V is 850 Wp*Ƞc*Ƞpv*Ƞv / 24 V =32 A.

This means that a 1600 VA MultiPlus will be needed (see table 6).

And the 3700 Wp array for the above average home would require a 8 kVA Quattro (or two 5 kVA

Multi’s in parallel, or three 3 kVA Multi’s in three phase configuration).

With the solar charge controller replaced by a PV inverter and the need for a much bigger

inverter/charger, the Hub-2 or -3 alternative is clearly the more expensive solution (and also less

efficient: see section 6).

Hub-2 or -3 may nevertheless be the preferred solution if:

- Intermediate power storage is added to a PV array plus PV inverter already installed.

- The relatively low PV voltage needed to supply the charge controller (max 150 V) and

therefore increased cable cross section is inconvenient due to long cable runs.

Notes:

- Even with some extra losses in the cabling from the PV array to the solar charge controller, Hub-1 may

still be the most efficient solution. See the MPPT 150/70 manual for calculation of DC cable losses.

- A mix of Hub-1 with Hub-2 or -3 is also possible.

- Sensitivity of the PV inverter to AC voltage variations (when power hungry loads are switched) may

reduce PV output (due to voltage glitches causing temporary shut downs of the PV inverter).

7.4. Base load plus other plug-in appliances (category 2 and 3 loads) powered with Hub-2

or -3 The more powerful inverter/charger (needed for battery charging, see section 7.3), could supply

category 2 and 3 loads without any or with only little help from the grid.

Battery capacity and the PV array would have to be increased by some 25% only to be completely

independent of the grid on the now familiar sunny summer day.

Self consumption would be close to 100%.

This achievement has its price: more PV, more battery capacity and a much more powerful

inverter/charger needed.

20

7.5. What about dark and rainy wintertime? During periods of bad weather (which could last days or even weeks), PV output can be dramatically

reduced to not more than a few % of its maximum summertime output, see table 4.

The PV array can be increased to provide sufficient output even during less sunny days, resulting in a

surplus to be fed back into the grid on sunny days, but “over sizing” by a factor of ten or more is

expensive and requires a large area for the PV array, and is therefore rather unusual.

And increasing battery capacity to ride through weeks of low or near zero output periods is

extremely expensive.

The common solutions to compensate for insufficient PV power are therefore:

- Use the power from the grid.

- Install a gas fired micro-CHP (micro combined heat and power) system. The micro-CHP will

provide the heat and the electric energy needed when the sun (and/or wind) leaves to

desire.

- Install a diesel engine powered generator.

21

8. The off-grid Hub

8.1. Micro-CHP

In densely populated areas, the wish to go off-grid can be fulfilled by adding a gas fired micro-CHP to

the system.

Generating heat with electricity is easy and the reverse, generating electricity with heat, is not. A

micro-CHP with a high electric efficiency is therefore preferable.

The few proven high efficiency micro-CHP systems (25% electricity, 75% heat) are all based on a

generator powered by a small long life internal combustion engine that runs on natural gas or

propane. Electricity produced from the generator is consumed directly or stored in the battery.

Simultaneously, the heat from the engine is captured to create thermal energy. The heat is used to

for central heating and/or to create hot water.

For more information see for example http://www.bhkw-infothek.de/

Stirling engine based systems have a lower electric efficiency (10-15% electricity, 90-85% heat) which

may result in excess heat production in a true off-grid system.

The fuel cell micro-CHP is still a promise for the future.

The electric power output of the micro-CHP should at the very minimum be equal to the average

power required. This is not difficult to achieve: even the wintertime average of the above average

home is 32,12 kWh per day (see table 8), which is less than 1,4 kW averaged over 24 hours.

If installed in combination with thermal and photovoltaic solar, the micro-CHP will be in use mostly

during the winter. The inverter-charger must be sized to power the complete home. As can be seen

from table 2, three to sixteen kVA will be needed.

The use of gas for cooking and clothes drying, and hot fill for the washing machine and dish washer is

recommended to reduce peak power required.

Battery capacity to cover one day of summertime electricity consumption will be sufficient as the

running periods of the micro-CHP can be synchronized with periods of peak electricity consumption.

The micro-CHP will run in parallel with the inverter charger, similar to the PV inverter of Hub-2 or -3.

Excess power will be used to recharge the battery, and insufficient power will be supplemented with

power from the battery (PowerAssist feature of the MultiPlus and Quattro inverter/charger).

The heat (engine heat plus exhaust heat) can be used for the home heating system and to heat the

boiler.

When both electric and heat output are fully used, the efficiency of a micro-CHP is around 98%.

(i. e. 98% of the caloric content of the gas burned is transformed in useful heat and electricity).

And with 40% of the electric output directly consumed by the load, the efficiency of the Hub, now

including the micro-CHP, will be around 86% in case of a Li-ion battery (see section 6.2).

Note:

In case of the two person energy conscious household daily warm water consumption will be 100 to 150 liter

(including hot fill dish washer and washing machine), which, when heated to increase temperature by 40°C,

requires 5 to 7 kWh of heat.

(specific heat capacity of water: C = 4,2 J/(g∙°K) ≈ 1,2 Wh/(liter∙°C),

see http://en.wikipedia.org/wiki/Heat_capacity )

At 25% electric efficiency, the micro-CHP will produce 25 / 75 = 0,33 kWh electric energy per kWh of heat.

With 6 kWh of heat needed, the electric output of the micro-CHP will be 2 kWh.

Counting with 15% loss (85% efficiency) in the hub, the available electric energy is 1,7 kWh.

22

Total daily electric energy consumption during wintertime is 7,5 kWh/day (see table 6).

This means that the micro-CHP will cover roughly 23% of electricity consumption of the two person energy

conscious household just when running to produce the required hot water.

If home heating is needed during winter, much more electric energy will be produced:

In The Netherlands the average consumption of natural gas per year for heating a free standing house is

2000 m³.

The caloric content of natural gas is 32 MJ/m³ and 1 kWh = 3,6 MJ.

The average daily energy need during the 6 months that space heating is required is:

32 MJ/m³ x 2000 m³ / 182 days = 352 MJ/day, or 97 kWh per day.

With 97 kWh of space heating needed per day, the daily electric output of the micro-CHP would be

97 x 0,33 = 32 kWh.

This happens to be the daily average wintertime daily electric energy consumption of the above

average home (see table 8).

Clearly, the micro-CHP is the solution of choice in colder areas were home heating is needed.

8.2. Diesel engine powered generator

In remote areas where power from the grid is not available or not reliable, the traditional solution is

to install a diesel engine powered generator (generator). The generator will be rated to cover the

highest power requirement expected.

The generator is much cheaper (per rated kVA) and easier to install and service than a micro-CHP, but

is noisy, smelly, less efficient (all the heat is wasted!), and needs frequent maintenance.

It also has a much shorter lifespan.

Note:

The traditional diesel powered generator can be modified to more closely resemble a gas powered micro-CHP,

mainly by modifications to reduce noise, to decrease maintenance and by adding an engine heat recuperation

system.

For more information see http://www.bhkw-infothek.de/

When running 24/7 or during most of the day, the traditional diesel engine powered generator

solution has two major drawbacks:

Maintenance and service life

Generators need frequent maintenance: oil change every 500 hours, belt replacement every 1000

hours, etc.

Service life of a good 1500 rpm generator is about 10.000 hours (= 3 years when running 24/7).

Fuel consumption at low load

A 10 kW generator will consume between 3 and 3,5 kg of fuel (= 3,7 to 4,4 liter) per hour when

powering a 10 kW load.

And at zero load it will still consume 1 kg/h! (see graph 1).

Running a generator 24/7 to power a home, with an average to peak load of less than 10% (see

table 3), is therefore an extremely inefficient and expensive solution, because of maintenance and

service life per kWh produced, and especially because of extremely high specific fuel consumption

(= fuel consumption per kWh produced).

23

Graph 1: Fuel consumption of three 1500 rpm diesel engine powered generators, max output

9-11 kW

As shown by graph 1, When the generator runs near maximum load (10 kW), specific fuel

consumption is around 0,3 kg per kWh.

When operating with 500 W load, specific fuel consumption is around 2 kg per kWh.

A 10 kW generator running 24/7 and burning on average 1kg/hr to power the average home will

consume some 9.000 kg (!) of fuel per year to produce the required 4.788 kWh (see table 70.

Without butane or propane gas for cooking and warm water, the all electric solution will increase

daily electric energy needed with 8 kWh to 21 kWh, and the average generator load would be close

to 1 kW. As can be seen from graph 1, this would only marginally increase fuel consumption, to some

10 ton/year.

And if a bigger generator is installed to cope with potentially higher peak loads, fuel consumption will

be even higher.

Graph 2 shows the absolute efficiency of three generators, rated at respectively 3,5 kW, 7 kW and

11 kW. Clearly absolute efficiency is around 25% at the most efficient load point. This means that

even when used at their most efficient load point, only 25% of the caloric content of the diesel fuel

(the caloric content of automotive diesel fuel is about 45,6 MJ/kg, or 12,7kWh/kg) is converted into

electric power. The remaining 75% is transformed into heat and is evacuated through the exhaust

and the engine cooling system.

Note:

For more information about generators see the VE Marine Generator Test, downloadable from

www.victronenergy.com

24

Graph2: Absolute efficiency of three representative generators

As can be seen from graph 2, generator efficiency reduces to 5-10% when operating with 500 W load.

Clearly, there is room for improvement here!

Option 1: adding a low power inverter/charger for nighttime low load periods only

A MultiPlus C 24/1600/40 for example.

The 1600VA inverter will power the base load. A sudden additional load such as a washing machine

will however cause the MultiPlus to go into overload protection mode, and the AC supply will

shutdown.

To prevent this, the generator must be on-line before any heavy load is switched on.

In practice, this option works well if the inverter/charger supplies the base load during the night, and

the generator is on during the day.

With the generator shut down during 8 hours every day, the yearly fuel consumption by the average

off-grid home would reduce to 10.000∙(24-8)/24 = 6.700 kg

Option 2: high power inverter/charger to substantially reduce generator size and running hours

Inverter power should be sufficient to support heavy loads until the generator is on-line.

A load dependent automatic generator start signal can be generated by the inverter/charger. In

addition a ‘battery discharged’ signal to start the generator can be provided by the inverter/charger,

a battery monitor or the BMS of the Li-ion battery. Completely automatic system operation is

therefore possible.

With reference to table 2 the combined ‘Multi/Quattro+generator’ rating should be 10 kW to 20 kW.

Now the generator will run only during periods of peak power demand and with help of PowerAssist

the inverter/charger can be set to operate the generator at its most efficient power point:

approximately 80% of its kW nameplate rating. Any excess power available will be used to charge the

battery, and insufficient power will supplemented with power from the battery.

The all electric average home (no butane or propane gas for cooking and warm water) will need on

average 21 kWh per day, and assuming 85% efficiency for the Hub with Li-ion battery, total power

needed would be 21/0,85 = 25 kWh

With a 10 kVA inverter/charger, generator power could for example be reduced to 7 kVA.

A 7 kVA generator with 4 to 5 kW load will run about 6 hours per day (if no solar/wind input).

Efficiency will be 25%, (0,3 kg of fuel per kWh) and yearly fuel consumption will be

0,3 kg/kWh x 25 kWh x 365 days = 2.700 kg.

Less than one third of the 24/7 solution.

With an OPzS battery, fuel consumption will be 0,3 kg/kWh x (21/0,75)kWh x 365 days = 3100 kg.

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8 9 10 11

Load (kW)

Ab

so

lute

eff

icie

ncy

(%

)

Onan e-QD MDKBL 7kW

Onan e-QD MDKBN 11kW

Paguro 4000 3,5kW

25

Ok, let´s add electric floor heating in the bathroom (3 kWh/day) and a swimming pool (no heating,

pump only: 5,6 kWh/day). This would increase yearly fuel consumption to 3.800 kg (Li-ion) or

4.300 kg (OPzS)

Solar and/or wind energy to further reduce running hours

This is of course the next step to further reduce running hours and fuel consumption. Hub-1 and Hub-

2 can both be used, but Hub-3 is not an option in this system because the PV inverter will shut down

when the generator is not running.

Three phase or single phase generator?

The problem with a (relatively) low power generator is balancing the loads over the three phases.

A 10 kVA generator for example can supply 3,3 kVA per phase.

How to connect the loads of the average home?

Connecting the washing machine, clothes dryer and dish washer each to a separate phase would

leave very limited power for other loads that could be on simultaneously.

Connecting the washing machine, clothes dryer and dish washer to one phase would be ok as long as

they are not used simultaneously. All other appliances could be distributed over the remaining two

phases.

In practice extreme situations where one phase is fully loaded or even over loaded and another

phase is operating at nearly zero load could often occur.

Wiring all loads to a single phase generator eliminates the load balancing problem.

26

Three phase pumps

Swimming pool and water well pumps are often three phase, but rated at not more than 3 kVA.

The solution is to add a variable frequency drive with single phase input. The frequency drive will

connect to a single phase supply and also eliminate the starting current peak.

Supplying heavy loads only when the generator is running

During overcast days or wintertime, when solar power has to be supplemented with power from the

generator, the generator should be running during periods of high power demand or, alternatively,

high power loads (water pumping, water heating) could be switched on when the generator is

running.

The Multi and Quattro inverter/chargers have a programmable second AC output for this purpose.

This output will connect the additional loads with a 1 minute delay to allow the generator to stabilize.

PowerAssist will take these additional loads into account (which would not be the case if connected

directly to the generator).

9. Definition: the 100% PV array and the 100% battery

From section 7.1.2: The solar irradiation on sunny summer days on south facing panels with axis tilt ≈ latitude is roughly

8 kWh/m²/day, and relatively independent of latitude.

With this (very) rough approximation it becomes possible to discuss PV output independently of

latitude and local climate, and adjust for local conditions with help of table 4.

With this approximation in mind it can be very clarifying to discuss PV output in units of sunny

summer day output (≈ 6 kWh per kWp as discussed in section 7.1.2) and, relating output to

consumption, to discuss PV in relation to energy consumption of a home, small office, workshop or

any situation where the daily electric energy need ranges from a kWh to 100 kWh.

We therefore will discuss the sunny summer day output of the PV array, and, similarly, the usable

storage capacity of the battery, in terms of the daily energy consumption.

A 100% PV array is defined as the array needed to cover 100% of the electric energy consumption of

a particular home or similar, on a sunny summer day.

A 50% PV array would cover 50% of energy consumption, on a sunny summer day.

Similarly, a 100% battery is a battery with sufficient usable storage capacity to store the energy

needed for one summer day.

10. Cost

10.1. Self-consumption: optimal storage capacity Self-consumption is a relatively new phenomenon. Its increasing popularity is driven by increasing

retail electricity prices and simultaneously decreasing feed in tariffs. Selling excess PV energy for, let’s

say 15 Eurocents per kWh at noon and buying it back in the evening for 25 Eurocents seems like a

bad deal. Better to store the excess for later use.

From a purely financial point of view, intermediate storage would be an interesting proposition if the

additional cost involved is lower than the cost incurred by selling electricity at low price and buying it

back later at a higher price.

27

A reasonably accurate financial justification for intermediate storage is not easily made. Except for

desert low latitude regions where the sun is shining every day, PV output will be subject to wild

variations from day to day and from season to season. Installing a PV array plus energy storage

covering 100% of the energy need on a sunny summer day (the 100% self consumption solution) is

certainly not optimal in high latitude regions: the battery will be oversized on overcast days and will

even be sitting idle on dark winter dater days when PV output is close to zero.

What can safely be stated is:

- The (financially) optimal storage capacity increases with increasing difference between the

retail electricity price and the feed in tariff.

- The optimal storage capacity decreases with latitude (and also depends on the local climate).

- The optimal storage capacity increases when system cost decreases.

As we have not (yet) devised a simple method to at least compute a rough approximation of optimal

intermediate storage capacity, we simply assume it to be around 30% of the sunny summer day

output of the PV array.

Another point is that self-consumption is required to ensure stability of the grid. A system with

limited storage capacity will however behave just like a system without intermediate storage once

the battery has been fully charged. On a sunny summer day, the battery may for example be fully

charged before noon and be of no use to attenuate fluctuations and limit feed back when needed

most.

One may therefore expect that in the near future a limit of one sort or another will be set to the

amount of power that may be fed back into the grid.

The limit may for example be that feedback should never exceed a percentage of the Pw rating of the

array. With a limit of 60%, for example, feedback power should not exceed 60% of installed PV

power.

A rough approximation of the energy that would be wasted or could better be stored in a battery as a

result of such a regulation is calculated below:

Assuming that the output from the array can be approximated by a half circle (starting at zero in the morning,

increasing to maximum output at noon and back to zero in late afternoon), the energy that must not be fed

back into the grid (or could be fed back later on the day) is represented by the green circular segment in

figure 5.

Figure 5: Limiting peak power fed back into the grid

28

With Pw = R = 1, d∙Pw is the maximum power that can be fed back into the grid.

The area A of the green circular segment is

A = (R²/2)∙(� – sin�) with � = 2arccos�

(see http://en.wikipedia.org/wiki/Circular_segment )

And the area of the half circle is C = (½)∙πR²

With these formulas the percentage that must be “shaved off” to limit feedback to d∙Pw for different values of

d can be calculated:

d = 0,6: A/C = 0,45/1,57 ≈ 0,3

d = 0,5: A/C = 0,61/1,57 ≈ 0,4

d = 0,4: A/C = 0,79/1,57 ≈ 0,5

(see http://www.handymath.com/cgi-bin/arc18.cgi )

If d = 0,6 (meaning that feedback into the grid should never exceed 60% of the Pw rating of the

array), the green area represents 30% of the half circle, and therefore at least 30% of the array

output must be absorbed by the load and/or stored in the battery.

In this case, assuming zero load, 100% system efficiency and a discharged battery in the morning,

battery storage could be reduced to 30% of the sunny summer day PV output while still satisfying

the (hypothetical) regulation for self consumption. The battery would then be used to store the

energy content of the green area while the remaining output from the solar array could be fed back

into the grid.

Note:

The alternative is to simply limit output of the grid inverter to 60% of Pw installed: no storage needed, and 30%

of the potential output of the array will be wasted on sunny summer days.

10.2. Off-grid: optimal storage capacity When a micro-CHP or generator is available, sufficient usable capacity to cover one full day is the

generally accepted norm.

If sun and/or wind are the only sources of energy, a combination of oversized PV and/or wind output

and oversized battery capacity (ie more than 100% as defined in section 9) will be needed to cover

periods with low PV/wind output.

10.3. Battery: lead acid or Li-ion, part 2 10.3.1. Lithium iron phosphate

A Lithium iron phosphate (LiFePO₄ or LFP) battery should preferably not be discharged to less than

20% of its rated capacity. It can be discharged about 2000 times to 20%, and it can be recharged at

high current up to nearly 100% (discharging regularly to less than 20% would reduce cycling

endurance disproportionately).

The useful Ah (and kWh) capacity is therefore 80% of its nominal rating.

10.3.2. Tubular plate lead acid

Tubular plate lead acid batteries, whether flooded (OPzS: Ortsfeste Panzerplatte mit

Spezialseparator) or gel (OPzV) are quite robust, and have proven to perform extremely well in off-

grid systems. This according to our own experience as well as several tests:

http://www.cres.gr/kape/publications/photovol/5BV-335.pdf

http://www.iea-pvps.org/index.php?id=9&eID=dam_frontend_push&docID=376

They can regularly be discharged to 30% of their C₁₀ capacity but charge efficiency becomes very low

and charge current acceptance reduces appreciably once the battery has been charged up to 80%.

These batteries should therefore be cycled between 80% and 30%, and regularly be recharged to the

full 100% to prevent sulfation.

29

A second reason to regularly fully recharge the OPzS battery is acid stratification.

(http://batteryuniversity.com/learn/article/water_loss_acid_stratification_and_surface_charge/ )

OPzS and OPzV batteries have a high internal resistance and therefore efficiency and available

capacity will be reduced substantially at high charge and discharge currents.

(for specifications see http://www.victronenergy.com/upload/documents/Datasheet%20-

%20OPzS%20batteries%20-%20rev%2004%20-%20EN.pdf )

10.3.3. Flat plate flooded and flat plate VRLA lead acid

Many different types of flat plate flooded and VRLA (valve regulated lead acid: gel and AGM)

batteries are available, and, generally speaking, the best ones are also the most expensive. According

to our experience they however all are less robust than tubular plate OPzV and especially OPzS

batteries in terms of cycling capability as well as sulfation risk.

Victron Energy sells a range of deep discharge flat plate VRLA (Gel and AGM) batteries which have

thicker plates than car batteries and than the lowest cost VRLA batteries. This results in reasonable

cycling performance, but does not eliminate sulfation risk.

(for specifications see http://www.victronenergy.com/upload/documents/Datasheet%20-

%20GEL%20and%20AGM%20Batteries%20-%20rev%2007%20-%20EN.pdf )

It is advisable limit discharge of these batteries to 50% of their C₂₀ capacity rating.

Similarly to tubular plate batteries, charge efficiency becomes very low and charge current

acceptance reduces appreciably once the battery has been charged up to 80%.

These batteries should therefore be cycled between 80% and 50%, and regularly be recharged to the

full 100% to limit sulfation.

A comparison between different batteries is given in the table below.

Notes:

1) As a result of their relative fragility, low cost flat plate AGM and gel batteries (and to a lesser extent

OPzV) in practice rarely reach the number of cycles (1500) that can be achieved in laboratory

conditions.

2) 0,1C means a charge and discharge current of 0,1 times the rated capacity in Ah. For a 100 Ah battery

this current would be 10 A

Table 5: Battery comparison

Flat plate Tubular plate Tubular plate Li-ion

AGM flooded (OPzS) gel (OPzV) LiFePO₄

Cost per rated kWh € 188 € 312 € 432 € 1.233

Usable capacity 30% 50% 50% 80%

Cost per usable kWh € 627 € 624 € 864 € 1.541

Efficiency @ I = 0,1C ² 80% 80% 80% 92%

Efficiency @ I = 0,5C 70% 60% 60% 92%

Cycle life @ 25°C 750 - 1500 ¹ 2500 2000 - 2500 2000

Volume per usable kWh 11,3 cm³ 15,4 cm³ 15,4 cm³ 8,7 cm³

Weight per usable kWh 82 kg 82 kg 82 kg 17 kg

Application seasonal use year round cycling year round cycling year round cycling

- off grid

holiday house

- homes, small offices,

workshops, etc


workshops, etc


workshops, etc

Can be installed yes no yes yes

in the living area

Regular full recharge yes yes yes no

needed

regular maintenance no yes no no

needed

30

10.4. The PV array The recent world wide reduction of feed in tariffs has resulted in over capacity instead of shortage of

PV panels and a dramatic price reduction.

As can be concluded from the table 6-8 the cost of the 100% PV array is around 20% of the total cost,

while the 100% Li-ion battery represents 70% of the total.

If the available (roof-) area is no constraint, the PV array can be enlarged substantially with limited

effect on total cost.

If local regulations reward feedback into the grid this should obviously be done. Doubling the area

would result in 50% self consumption on a sunny summer day, and up to 45 degrees latitude

sufficient energy would be harvested during most of the year to power the home (depending on local

climate, see table 4).

And even if local regulations do not reward or even prohibit feedback into the grid, it could be

advantageous to have some excess capacity on sunny summer days in order to harvest more energy

on other days.

10.5. Examples: cost of the major components The tables below detail the options as discussed for self consumption, with a cost indication of each

of the major components, based on Victron Energy recommended consumer prices.

10.5.1. To summarize:

Three homes were discussed, each detailed in one of the following tables: The two person energy conscious household

The average home

The above average home

With these three examples the requirements and cost for other applications, such as a small office or

workshop, can easily be defined.

The spreadsheet with which the tables were created can be downloaded from

www.victronenergy.com.

For each home three types of loads were identified: Category 1: the base load, mainly consisting of low power appliances that are switched on permanently or

during long periods of time every day. The base load therefore has a low kW/kWh ratio and can efficiently be

powered by a battery plus low power inverter. The base load is by far the largest electricity consumer in the

home.

Category 2: plug-in appliances, which can easily be moved from one socket to another (especially the vacuum

cleaner) and are used during short periods of time. These loads have a high kW/kWh ratio but cannot easily be

separated from the base load.

Category 3: non movable loads that are always connected to the same socket. It is sometimes possible to

bypass the Hub and connect these loads directly to the grid, thereby reducing peak power required. Peak

electric power required can also be reduced by using thermal solar and/or gas instead of electricity for heating

purposes.

With a load management system several category 3 loads can be switched on when the sun is shining, thereby

increasing self consumption without additional battery storage capacity needed.

31

10.5.2. The first three tables (table 6-8) reflect the examples as discussed in section 7

• PV array:

The PV array has been sized to harvest sufficient energy to supply 100% of the required energy by one

or more load categories on a sunny summer day.

The rationale behind this choice is that:

- On a sunny summer day the energy harvested from a solar panel is about the same, all over the

world. The tables are therefore universally applicable.

- With sufficient battery storage, self consumption will be close to 100%, even on a sunny summer day.

The consequence is that on all other days of the year the amount of energy harvested will not be

sufficient to cover consumption. Additional energy has to be supplied by the grid. Self consumption,

however, will always be 100%.

• Li-ion battery:

The Li-ion battery has been sized to store the energy required for one or more load categories during

one summer day. Year round 100% self consumption is therefore ensured. But the battery will be over

dimensioned on all those days of the year when less energy is harvested.

The Li-ion battery clearly is by far the most expensive part of the system.

10.5.3. Table 9 to 11: the three homes, with OPzS battery

• OPzS Battery:

In these tables the Li-ion battery has been replaced by an OPzS battery, again sized to store the energy

required for one or more load categories on a sunny summer day. Year round 100% self consumption

is therefore ensured. But the battery will be over dimensioned on all those days of the year when less

energy is harvested. The nominal energy storage capacity is higher because useful capacity reduces to

50% in comparison to 80% in case of the Li-ion battery (see section 10.3.3).

• PV array:

The PV array has again been sized to harvest sufficient energy to supply 100% of the required energy

by one or more load categories on a sunny summer day.

The slightly larger array Pw reflects the lower efficiency of the OPzS battery compared to Li-ion.

Total system cost is nevertheless much lower than the Li-ion option.

With 100% battery storage and 100% PV, the column labeled category 1+2+3 in tables 6 to 11 is

representative for an off-grid situation with sufficient PV power to avoid running the micro-CHP or

generator on sunny summer days. Running hours of the micro-CHP or generator can be further

reduced by over sizing the PV array and/or battery.

10.5.4. Table 12 to 14: Battery energy storage reduced to 30% of PV output

Table 12 to 14 each consist of 5 sub tables which summarize cost for several battery and PV

solutions.

The first sub tables (a) are a concentrated version of table 6 to 8.

The Li-ion battery and the PV array are both sized at 100%.

The following three sub tables (b, c and d) are based on a self consumption regulation stipulating that

at most 60% of the Wp power of the array may fed back into the grid. As shown in section 10.1,

battery storage can then be reduced to approximately 30% of the sunny day kWh array output.

In sub tables b the size of the PV array has been kept at 100% and battery storage was therefore

reduced to 30%.

In sub tables c and d the PV array has been increased to respectively 200% and 300%, and battery

storage has been increased accordingly.

In sub tables e the PV array was again sized at 300% but the Li-ion battery has been replaced with a

OPzS battery, sized at 100%.

32

Note:

Regarding system efficiency the matter becomes complicated as soon as the battery becomes too small to

store the daily energy harvest from the sun (or wind), as will be the case when battery storage is reduced to

30% of the sunny day PV array output. In that case some of the potential energy harvest will be wasted (if

feedback into the grid is not possible), or will be directly consumed by the load (if there is a load), or will be fed

back into the grid, bypassing the battery.

Direct feedback into the grid does increase efficiency (no losses due to battery cycling), and simultaneously

decreases self consumption.

Note:

Not every day is a sunny summer day, in most regions. When less energy is harvested, relatively more energy

will “go through” the battery, decreasing efficiency but increasing self consumption.

To keep things simple the sub tables have been created assuming that 100% of the harvested energy goes

through the battery. This assumption may be close to reality in high latitude areas with few sunny days, but is

pessimistic (with regard to efficiency) in case of sunny low latitude areas.

If we take Sevilla (Spain) for example, table 4 shows that the average annual output is 74% of the sunny

summer day output. If the battery is sized to store 30% of the sunny summer day PV output, roughly 74% -

30% = 44% will be fed back into the grid and/or supply a load, bypassing the battery and the related losses (8%

in case of Li-ion and approximately 20% in case of lead acid).

Note:

Battery capacity will slowly reduce with time. The generally accepted end-of-life capacity is 80% of the name

plate capacity. In order to have the required capacity still available when the battery reaches end-of-life, a new

battery should be overrated by a factor 1/0,8 = 1,25. This factor is not included in the energy storage capacity

as calculated in the following tables.

33

Table 6: The two person energy conscious household

100% Li-ion battery and 100% PV

The column labeled Category 1+2+3 includes the non movable loads (= appliances always connected

to the same socket).

In this example the non movable loads consume, on average, only 350 Wh per day.

This is because the following choices were made:

- best in class hot fill washing machine

- gas heated clothes dryer

- hot fill dish washer

- gas fired cooktop

- gas fired central heating and boiler

Two person energy conscious household Category 1 Category 1+2 Category 1+2+3

Li-ion battery (base load) (plus pluggable loads) (the complete home)

Electric energy consumption

Summertime S 4,37 5,73 6,08 kWh

Wintertime W 5,75 7,11 7,46 kWh

Annual Ey = 365*(S+W)/2 1801 2286 2410 kWh

Li-ion battery with sufficient storage capacity to store 100% of the daily summertime electric energy consumption

Energy storage capacity S/(0,80*0,94) 5,81 7,62 8,09 kWh

Nominal voltage 24 24 24 V

Ah storage capacity Esc/Nv 242 317 337 Ah

Cost 1233 €/kW € 7.165 € 9.395 € 9.969

Solar array with sufficient output to supply 100% of the load during a sunny summer day

Required daily Hub output S* 1 4,37 5,73 6,08 kWh/day

Required daily PV output RdHo/0,85 5,14 6,74 7,15 kWh

Array Wp RdPVo/6 857 1124 1192 Wp

Cost 2,19 €/Wp € 1.877 € 2.461 € 2.611

Hub-1

Efficiency solar charge

controller + DC cablesȠm*Ƞw 96 96 96 %

Max. Charge current Ƞm*Ƞw*Awp/Nv 34 45 48 A

Solar charge controller MPPT 70/50 € 260 MPPT 70/50 € 260 MPPT 70/50 € 260

Max. Load L 660 2660 2660 W

Inverter/charger Multi Multi Multi

GridAssist needed 24/2000/50 € 1.454 24/2000/50 € 1.454

GridAssist not needed 24/1200/25 € 969 24/3000/70 24/3000/70

Hub-1: cost of the major components € 10.271 € 13.570 € 14.294

Hub-2 or -3

PV inverter 1,5 kW € 1.149 1,5 kW € 1.149 1,5 kW € 1.149 kW

Efficiency PV inverter +

Inverter/chargerȠc*Ƞpv*Ƞv 90 90 90 %

Max. Charge current Ƞc*Ƞpv*Ƞv*Awp/Nv 32 42 45 A

Max. Load L 660 2660 2660 W


GridAssist not needed 24/1600/40 € 1.163 24/3000/70 € 2.180 24/3000/70 € 2.180

Hub-2 or -3: cost of the major components € 11.354 € 15.185 € 15.909

34

Table 7: The average home


The column labeled Category 1+2+3 includes non movable loads (= appliances always connected to

the same socket) which, in this example, consume on average 2050 Wh per day:

- washing machine with electric water heater

- clothes dryer with electric heater

- dish washer with electric water heater

- gas fired cooktop


The average home Category 1 Category 1+2 Category 1+2+3





Annual Ey = 365*(S+W)/2 3475 4058 4788 kWh





Cost 1233 €/kW € 13.740 € 16.429 € 19.790





Cost 2,19 €/Wp € 3.598 € 4.303 € 5.183

Hub-1





Max. Load L 1305 3305 3805 W


GridAssist needed 48/3000/35 € 2.180

GridAssist not needed 24/2000/50 € 1.454 48/3000/35 € 2.180 48/5000/70


Hub-2 or -3

PV inverter 2 kW € 1.393 2 kW € 1.393 2,8 kW € 1.670 kW




Max. Load L 1305 3305 3805 W




35

Table 8: The above average home







- electric induction cooktop


The above average home Category 1 Category 1+2 Category 1+2+3





Annual Ey = 365*(S+W)/2 7487 8170 10698 kWh





Cost 1233 €/kW € 31.087 € 34.235 € 45.877





Cost 2,19 €/Wp € 8.142 € 8.966 € 12.015

Hub-1




Solar charge controller MPPT 150/75 € 720 2*MPPT 150/75 € 1.440 2*MPPT 150/75 € 1.440

Max. Load L 2560 4560 10560 W



GridAssist not needed 48/3000/35 € 2.180 48/5000/70 48/10000/140


Hub-2 or -3

PV inverter 5 kW € 2.554 5 kW € 2.554 8 kW € 4.000 kW




Max. Load L 2560 4560 10560 W




36

Table 9: The two person energy conscious household

100% OPzS battery and 100% PV

The column labeled Category 1+2+3 includes the non movable loads (= appliances always connected

to the same socket).

In this example the non movable loads consume, on average, only 350 Wh per day.

This is because the following choices were made:

- best in class hot fill washing machine

- gas heated clothes dryer

- hot fill dish washer

- gas fired cooktop


Note:

In order to reduce the number of battery cells and increase Ah per cell, a lower DC system voltage is sometimes

preferred.

Two person energy conscious household Category 1 Category 1+2 Category 1+2+3

OPzS battery (base load) (plus pluggable loads) (the complete home)




Annual Ey = 365*(S+W)/2 1801 2286 2410 kWh

OPzS battery with sufficient storage capacity to store 100% of the daily summertime electric energy consumption




Cost 312 €/kW € 2.901 € 3.804 € 4.036





Cost 2,19 €/Wp € 2.127 € 2.789 € 2.959

Hub-1





Max. Load L 660 2660 2660 W



GridAssist not needed 24/1200/25 € 969 24/3000/70 24/3000/70


Hub-2 or -3

PV inverter 1,5kW € 1.149 1,5kW € 1.149 1,5kW € 1.149 kW




Max. Load L 660 2660 2660 W




37

Table 10: The average home

100% OPzS battery and 100% PV






- gas fired cooktop


Note:


preferred.

The average home Category 1 Category 1+2 Category 1+2+3





Annual Ey = 365*(S+W)/2 3475 4058 4788 kWh





Cost 312 €/kW € 5.563 € 6.652 € 8.012





Cost 2,19 €/Wp € 4.078 € 4.876 € 5.874

Hub-1





Max. Load L 1305 3305 3805 W


GridAssist needed 48/3000/35 € 2.180

GridAssist not needed 24/2000/50 € 969 48/3000/35 € 2.180 48/5000/70


Hub-2 or -3

PV inverter 2kW € 1.393 2kW € 1.393 2,8 € 1.670 kW




Max. Load L 1305 3305 3805 W




38

Table 11: The above average home







- electric induction cooktop


Note:


preferred.

The above average home Category 1 Category 1+2 Category 1+2+3





Annual Ey = 365*(S+W)/2 7487 8170 10698 kWh





Cost 312 €/kW € 12.586 € 13.861 € 18.574





Cost 2,19 €/Wp € 9.227 € 10.162 € 13.617

Hub-1




Solar charge controller MPPT 150/75 € 720 2*MPPT 150/75 € 1.440 2*MPPT 150/75 € 1.440

Max. Load L 2560 4560 10560 W



GridAssist not needed 48/3000/35 € 2.180 48/5000/70 48/10000/140


Hub-2 or -3

PV inverter 5 kW € 2.554 5 kW € 2.554 8 kW € 4.000 kW




Max. Load L 2560 4560 10560 W




39

Table 12 The two person energy conscious household

Two person energy conscious household Category 1+2 Category 1+2+3

Li-ion battery 100% 7,62 kW € 9.395 69% 8,09 kW € 9.969 70%

PV array 100% 1.124 Wp € 2.461 18% 1.192 Wp € 2.611 18%

Solar charge controller MPPT 70/50 € 260 2% MPPT 70/50 € 260 2%

Inverter/charger 24/2000/50 € 1.454 11% 24/2000/50 € 1.454 10%

Hub-1: cost of the major components € 13.570 100% € 14.294 100%

PV inverter 1,5 kW € 1.149 8% 1,5 kW € 1.149 8%


Hub-2 or -3: cost of the major components € 15.185 112% € 15.909 111%


PV array 100% 1.124 Wp € 2.461 35% 1.192 Wp € 2.611 36%




PV inverter 1,5 kW € 1.149 16% 1,5 kW € 1.149 16%




PV array 200% 2.247 Wp € 4.921 37% 2.384 Wp € 5.222 37%




PV inverter 2,8 kW € 1.670 12% 2,8 kW € 1.670 12%




PV array 300% 3.371 Wp € 7.382 38% 3.576 Wp € 7.832 38%




PV inverter 4 kW € 2.241 11% 4 kW € 2.241 11%



OPzS battery 100% 12,19 kW € 3.804 25% 12,94 kW € 4.036 26%

PV array 300% 3.820 Wp € 8.366 56% 4.053 Wp € 8.877 56%




PV inverter 4 kW € 2.241 15% 4 kW € 2.241 14%



Table 12a

Concentrated version

of table 6

Table 12b

Li-ion battery optimized

for self consumption

(see section 10.1)

Table 12c



with 200% PV array

Table 12d



with 300% PV array

Table 12e

OPzS battery optimized


with 300% PV array

40

Table 13 The average home

Average home Category 1+2 Category 1+2+3


PV array 100% 1.965 Wp € 4.303 18% 2.367 Wp € 5.183 19%




PV inverter 2 kW € 1.393 6% 2,8 kW € 1.670 6%




PV array 100% 1.965 Wp € 4.303 35% 2.367 Wp € 5.183 37%




PV inverter 2 kW € 1.393 11% 2,8 kW € 1.670 12%




PV array 200% 3.929 Wp € 8.605 38% 4.733 Wp € 10.366 39%

Solar charge controller 2*MPPT 150/70 € 1.440 6% 2*MPPT 150/70 € 1.440 5%



PV inverter 4 kW € 1.670 7% 5 kW € 2.554 10%




PV array 300% 5.894 Wp € 12.908 36% 7.100 Wp € 15.549 37%




PV inverter 6 kW € 2.800 8% 8 kW € 4.000 10%




PV array 300% 6.680 Wp € 14.629 53% 8.047 Wp € 17.622 55%




PV inverter 8 kW € 4.000 15% 10 kW € 5.000 16%



Table 13a

Concentrated version of

table 7

Table 13b



(see section 10.1)

Table 13c



with 200% PV array

Table 13d



with 300% PV array

Table 13e



with 300% PV array

41

Table 14 The above average home

The above average home Category 1+2 Category 1+2+3


PV array 100% 4.094 Wp € 8.966 19% 5.486 Wp € 12.015 19%




PV inverter 5 kW € 2.554 5% 8 kW € 4.000 6%




PV array 100% 4.094 Wp € 8.966 38% 5.486 Wp € 12.015 40%




PV inverter 5 kW € 2.554 11% 8 kW € 4.000 13%




PV array 200% 8.188 Wp € 17.932 40% 10.973 Wp € 24.030 41%




PV inverter 10 kW € 5.000 11% 12 kW € 6.000 10%




PV array 300% 12.282 Wp € 26.898 37% 16.459 Wp € 36.045 38%


Inverter/charger 3*48/5000/70 € 8.721 12% 3*48/5000/70 € 8.721 9%


PV inverter 15 kW € 7.500 10% 20 € 10.000 11%




PV array 300% 13.920 Wp € 30.485 54% 18.653 Wp € 40.851 57%




PV inverter 15 kW € 7.500 13% 20 € 10.000 14%



Table 14a

Concentrated version of

table 8

Table 14b



(see section 10.1)

Table 14c



with 200% PV array

Table 14d



with 300% PV array

Table 14e



with 300% PV array

Victron Energy B.V. | De Paal 35 | 1351 JG Almere | The Netherlands

General phone: +31 (0)36 535 97 00 | Fax: +31 (0)36 535 97 40

E-mail: [email protected] | www.victronenergy.com

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